1. Field of the Invention
This invention relates to a method for the preparation of high density igniter granules, and in particular to the substantially water-free preparation of high density igniter granules, which igniter granules are especially suitable for use in waterless extrusion applications.
2. Description of Related Art
Among the various applications in which thermoplastic elastomer-coated igniters extruded from granules may be employed are, by way of example, air bag assemblies, pyrotechnic ordinances, flares, thrust decoys, and solid propellant rocket motors. Especially suited for these applications are igniter compositions comprising boron/potassium nitrate (B/KNO.sub.3) as the fuel/oxidizer ingredients. When ignited, the igniters initiate combustion of the larger specified gas generant composition.
Processes for preparing igniter granules are disclosed in U.S. Pat. Nos. 4,405,534 to Deisenroth and 5,565,651 to Kim, the disclosures of which are incorporated herein by reference to the extent that these disclosures are compatible with this invention. The Deisenroth patent discloses a process in which a crystalline water-insoluble explosive is incorporated into a fully reacted aqueous polyurethane dispersion that is substantially free of organic solvent to form a uniform mixture. The mixture is then coagulated to form granules, which are dried. The process disclosed in the Kim patent differs from that of Deisenroth, yet similarly involves the use of an aqueous solution.
The presence of water in the above-discussed processes imparts the processes with several drawbacks. In particular, water-sensitive materials that are soluble in water, such as B/KNO.sub.3, cannot be effectively granulated by these water-intensive processes. Additionally, other water-sensitive materials, such as magnesium compounds, may produce volatile and explosive by-products when exposed to water.
A water-free process for preparing igniter granules is disclosed in U.S. Pat. No. 4,979,999 to Briere, the disclosure of which is incorporated herein by reference to the extent that it is compatible with this invention. According to the process disclosed in Briere, a binder is first dissolved in a solvent to form a binder solution. Ignitable magnesium particles are then added to the binder solution, and thereafter a non-solvent is introduced into the solution. However, the addition of ignitable particles to the binder solution before introducing the non-solvent to the ignitable particles causes a paste to form. Drawbacks associated with paste include the need to subject the paste to additional processing steps and equipment, such as an oven heater and a mesh sieve. Moreover, the igniter granules produced by this conventional technique typically are characterized by poorly distributed coatings, irregular shapes and a wide distribution of granule sizes, typically between approximately 0.1 mm to 1 mm. The lack of uniformity in granule coating, configuration, and dimension makes the igniter granules more sensitive to impact, friction, and electrical static discharge (ESD).
Thus, it would be a significant advancement in the art to provide a substantially water-free method for making uniformly coated extrudable igniter granules of substantially uniform shape and size without the need for either oven drying to evaporate water or complicated distillation processes to distill organic solvents.